1. Field of the Invention
The present invention generally relates to the demodulation of OFDM (Orthogonal Frequency Division Multiplexing) digital modulated signals, and more particularly, the present invention relates to an equalizer which may be utilized in the demodulation of OFDM digital modulated signals.
A claim of priority is made to Japanese patent application no. 2007-262150, filed Oct. 5, 2007, the entirety of which is incorporated herein by reference.
2. Description of Related Art
In broadcasting systems, such as terrestrial digital broadcasting (ISDB-T), OFDM modulation techniques have been adopted in view of their relative immunity to multi-path interference. In OFDM modulation, a scattered pilot method may be utilized in which modulation is executed by scattering pilot symbols along frequency and time directions as amplitude and phase references in the data symbols, and demodulation is executed by estimating propagation-path characteristics (channel estimation) using the pilot symbols and compensating the amplitude and phase of the received signal according to the estimated propagation-path characteristics (that is, removing distortion of the propagation-path)
Propagation-path estimation using channel and symbol filters is generally known in the art. This technology extracts information for compensation of the sub-carrier of pilot symbols using already-known pilot symbol and generates the information for compensation of sub-carriers not including the pilot symbols by filtering. Consequently, noise components included in the sub-carrier of pilot symbols pass through the filter, and spread to neighboring channels, and then an error in propagation-path estimation occurs. A method for carrying out more precise propagation-path-characteristics estimation not only by using pilot symbols included in a current OFDM symbol but also by holding a pilot symbols in past OFDM symbols so as to reduce intervals between sub-carries including pilot symbols can be used in order to reduce the above error in propagation-path-characteristics estimation. However, since there is a difference between the propagation-path characteristics received by the past OFDM symbols and the propagation-path characteristics received by the current OFDM symbol in the circumstances where the propagation-path state is changed rapidly, the above error in propagation-path estimation can become rather large.
In addition, there is an already-known propagation path estimator for estimating directly a complex gain and a delay time in a propagation path. The estimator calculates a complex gain and delay time having the smallest difference between a product of complex gain and delay time in a propagation path estimated by a received pilot symbol signal obtained by using the already-known pilot symbol and a product of complex gain and delay time in a propagation path estimated by the already-known pilot symbol, and the estimator transforms the above complex gain and delay time to frequency components and multiplies the frequency components by the original multiplicative inverse. Since the delay time calculated by the above method is not a real delay time and is calculated based on discrete time used in digital signal processing, there is an error in the delay time, and a plurality of complex gains occurs at a plurality of delay times as errors. Therefore, in an estimation of a real propagation-path caused by plurality of delay paths, complex gains occurs at more delay time positions than the number of the delay paths occurs in reality. Subsequently, the error in propagation-path estimation becomes larger. Additionally, the conventional method calculates complex gains and delay times in the number of detectable delay paths. First, the largest complex gain and delay time is obtained among the detectable complex gains and delay times, and generates a product of the already-known pilot symbol signals and the above obtained complex gain and delay time to subtract the received pilot signals by the above generated product. Secondly, the procedure is moved to detection of the second strongest delay path and the same calculation is being carried out. Consequently, repeating times more than the number of delay paths necessary to be detected becomes necessary, and then calculation amount becomes very large. Furthermore, since the number of delay paths in a real transmission cannot be already known, largish repeating time is necessary to be used.
In addition, according to the paragraph 0057 of the Japanese Patent Application Laid-Open Publication No. 2006-262039, a propagation-path estimation apparatus is publicly known, as follows. The propagation-path estimation apparatus conducts inverse Fourier transform using only the received pilot symbols, generates a value as a threshold by subtracting the predetermined power from the largest power represented by the calculated complex gains, and uses only a complex gain having a power larger than the threshold as information for the propagation-path estimation.
However, according to the propagation-path estimation apparatus described in Japanese Patent Application Laid-Open Publication No. 2006-262039, there is a problem as follows, too. In the case where a fading phenomena caused by a multi-path in a propagation path between a base station and a mobile station, a variation in amplitudes of required receiving radio wave arises as time passes, and both states of larger and smaller ratios of the noise component to the required radio wave are mixed in the time domain. In an example shown in FIG. 13, during a period T1, the noise ratio to the required radio wave becomes smaller, and during another period T2, the noise ratio to the required radio wave becomes larger. However, in the case where a fading phoneme of FIG. 13 occurs after a value has been determined as a threshold by subtracting the predetermined power from the largest power, as in the before-mentioned technology according to Japanese Patent Application Laid-Open Publication No. 2006-262039, it becomes more possible that most of the noise components passes during another period T2, even if the required radio wave can be received while restraining most of the noise components from passing during a period T1. Consequently, the precision of the estimation becomes lower and the received signal cannot be demodulated stably.